<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#block1 {
 height: 20px;
 margin-bottom: 20px;
 background-color: green;
}
#margin-only {
 margin-top: 30px;
 margin-bottom: 40px;
}
#block2 {
 height: 20px;
 margin-top: 10px;
 background-color: green;
}
</style>
</head>
<body>
<div id="block1"></div>
<div id="margin-only"></div>
<div id="block2"></div>
</body>
</html>
